!UPSTREAM: i965: Add an assertion to brwProgramStringNotify.

driver->ProgramStringNotify is only called for ARB programs, fixed
function vertex programs, and ir_to_mesa (which isn't used by the i965
back-end).  Therefore, even after geometry shaders are added,
brwProgramStringNotify should only ever be called with a target of
GL_VERTEX_PROGRAM_ARB or GL_FRAGMENT_PROGRAM_ARB.

This patch adds an assertion to clarify that.